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Eddy covariance (EC) flux measurements of the atmosphere/surface exchange ofgases over an urban area are a direct way to improve and evaluate emissionsinventories, and, in turn, to better understand urban atmospheric chemistryand the role that cities play in regional and global chemical cycles. Aspart of the MCMA-2003 study, we demonstrated the feasibility of using eddycovariance techniques to measure fluxes of selected volatile organiccompounds (VOCs) and CO2 from a residential district of Mexico City(Velasco et al., 2005a, b). During the MILAGRO/MCMA-2006 field campaign, asecond flux measurement study was conducted in a different district ofMexico City to corroborate the 2003 flux measurements, to expand the numberof species measured, and to obtain additional data for evaluation of thelocal emissions inventory. Fluxes of CO2 and olefins were measured bythe conventional EC technique using an open path CO2 sensor and a FastIsoprene Sensor calibrated with a propylene standard. In addition, fluxes oftoluene, benzene, methanol and C2-benzenes were measured using avirtual disjunct EC method with a Proton Transfer Reaction MassSpectrometer. The flux measurements were analyzed in terms of diurnalpatterns and vehicular activity and were compared with the most recentgridded local emissions inventory. In both studies, the results showed thatthe urban surface of Mexico City is a net source of CO2 and VOCs withsignificant contributions from vehicular traffic. Evaporative emissions fromcommercial and other anthropogenic activities were significant sources oftoluene and methanol. The results show that the emissions inventory is inreasonable agreement with measured olefin and CO2 fluxes, whileC2-benzenes and toluene emissions from evaporative sources areoverestimated in the inventory. It appears that methanol emissions frommobile sources occur, but are not reported in the mobile emissionsinventory

Eddy covariance flux measurements of pollutant gases in urban Mexico City